The subject invention concerns a safety device, which is intended to be used particularly in roll-up doors for the purpose of causing reversal of the closing movement of the door when an object is obstructing its path of movement. The safety device is of the kind comprising at least one signalling means which is arranged to emit a beam of light, a beam of sound or another kind of signalling beam and which is supported by the door leaf some distance from the door leaf closing edge.
One example of prior-art safety devices for doors is formed with a sealing strip which is positioned at the lower edge of the door leaf. The sealing strip consists e.g. of a hollow strip of soft rubber, the ends of which are sealed to enclose a quantity of air in the interior of the strip. Should the door leaf hit an obstacle, while moving downwards into its fully closed position, the air inside the sealing strip is compressed, resulting in actuation of a diaphragm valve to reverse the closing movements of the door leaf. This construction does, however, suffer from the disadvantage that it is comparatively fragile and sensitive, since the slightest leakage makes the device inoperative.
Considering that doors of this kind are frequently damaged, for instance by trucks and other vehicles passing through the doorway, the use of highly sensitive sealing strips is not very advisable. Another serious drawback inherent in this prior-art structure is that reversal of the door leaf movements is not initiated until the leaf hits the object which is obstructing its path of movement.
In another prior-art structure the problem is solved thereby that instead of enclosing an air cushion inside the lower sealing strip a photoelectric cell is used the beam of which is arranged to pass through the strip. Although this structure is leakage-proof, it still depends on deformation of the strip for the initiation of the reversal of the door movements.
In accordance with a third prior-art structure which suffers from disadvantages identical with those inherent in the first two prior-art structures discussed in the aforegoing two copper rails are mounted on the strip which rails are kept apart by a spring of a non-conductive material.
Finally, it is also known to use photoelectric cells which are positioned closely above the floor level. Although this structure does not depend on contact between the door leaf and the obstructing object for its operation persons may easily climb over the beam of the photoelectric cell and the forks of fork-lift trucks may pass over it, and so the device will fail to operate. This prior-art structure could be combined with any one of the structures referred to in the aforegoing which become operative to reverse the closing movement of the door leaf upon contact with an obstructing object. However, combinations of this kind suffer from the disadvantage that also in such cases contact is required between the door leaf and the obstructing object to make the safety device operative, in addition to which installation thereof is comparatively expensive because of the necessity to double the sensing means.